Glow discharge analytical instrument

Abstract: A non-conductive isolator (10) for supporting a sample (137) to be analyzed by radio frequency energy induced glow discharge techniques has a conductive ring (19) for coupling a source of radio frequency energy to the same surface of the sample which is in direct contact with the induced glow discharge. An adapter kit for converting a direct current glow discharge analysis apparatus to a radio frequency energy induced flow discharge is described as well as the method of analysing non-conductive and conductive samples using a source of RF energy coupled to the same sample surface being analyzed.

Description: Commercial glow discharge atomic emission and mass spectrometer systems are generally limited to the analysis of conductive solid samples. In an attempt to develop glow discharge techniques for the analysis of non-conductive samples, research efforts have been made in the use of radio frequency powered sources for generating the glow discharge; see, for example, US-A-5006706, US-A-5086226, US-A-5028133 and US-A-3543077. The systems described in all of these specifications rely on coupling the RF power through the sample, the sample is analyzed on one side and the RF power is applied to the opposite side. This method of analysis is limited in that the analytical signals will vary with elemental concentration (the desired relationship) and also with sample thickness (not a desired factor). This latter factor is due to the efficiency with which the RF power can be coupled to the analytical zone (portion of the sample that will be analyzed). This factor becomes important when attempting to compare analytical signals from different samples (which may not have the same thickness) . As a result, the method is limited to samples of the same thickness or degraded analytical results are obtained from samples of differing thicknesses. The invention is set out in the independent claims and preferred features are included in the dependent claims. Broadly speaking, in application of the present invention, an RF powered glow discharge source applies the RF power to the same side of the sample upon which the analytical zone is located. Thus, the thickness of a sample no longer affects the analytical signal. Therefore, samples of any thickness may be analyzed freely. The invention may be carried into practice in various ways but two constructions will now be described by way of example together with their mode of operation and with reference to the accompanying drawings, in which: Fig. 1 is a plan view of the non-conductive isolator showing the electrical contact ring; Fig. 2 is a sectional view of Fig. 1 taken along the plane II-II; Fig. 3 is a plan view of a cathode with internal water cooling; Fig. 4 is an exploded sectional view showing isolator spaced from a cathode taken along plane IV-IV in Fig. 3; Fig. 5 is a schematic sectional view of a prior art DC glow discharge source with cross-hatching omitted for clarity; Fig. 6 is a schematic sectional view of the RF glow discharge source of the present invention with cross-hatching omitted for clarity; Fig. 7 is a schematic view of the apparatus of Fig. 6 with a vacuum source and spectrometer shown in block diagram form with cross-hatching omitted for clarity; Fig. 8 is a plan view of an alternative embodiment of a non-conductive interface; Fig. 9 is sectional view of Fig. 8 taken along plane IX-IX in Fig. 8; Fig. 10 is a plan view of the insulator according to Fig. 8 with a retainer removed; Fig. 11 is a sectional view of Fig. 10 taken along plane XI-XI in Fig 10; Fig. 12 is a top plan view of a glow-discharge lamp including the interface according to Fig. 8; and Fig. 13 is a sectional view of the interface according to Fig. 12 taken along plane XIII-XIII in Fig. 12. Referring to Figs. 1 and 2, an interface assembly 9, including non-conductive isolator 10, is shown which can be attached to the cathode plate in a Grimm-type glow discharge lamp. Isolator 10 can be made of a suitable non-conductive material, such as ceramic, and is approximately 76 mm (3 inches) in diameter and 9.5 mm (3/8 of an inch) thick at the widest central portion. The isolator has a central aperture 11 which is bounded by a raised edge 13. A groove 15 is provided adjacent edge 13 into which an elastic sealing member such as an O-ring 17 is positioned. A preferably circular, electrical contact member 19 surrounds the raised portion of the isolator and is recessed into the insulating material forming the isolator. Electrical contact member 19 has a raised annular ring portion 21. The top face 22 of the annular ring projects outwardly approximately 0.125 mm (0.005 inch) beyond the top face of edge 13 to provide a uniform electrical contact with the face of the sample. While a circular electrical contact member is preferred, the contact can be of other configurations such as an aperture in a rectangular contact pad or multiple contact pads spaced about the aperture in the isolator. The important criterion is that the contact provides a substantially uniform contact with the glow discharge region. A radial, recessed channel 23 is provided in the surface of the isolator for an electrical contact to be inserted to make contact with conductive ring 19 at a slot 25. A copper wire 27 (Fig. 2) can be inserted into groove 23 with the end of the wire either silver soldered to contact 19, or connected by a suitable threaded member, such as a screw (not shown). On the opposite face of isolator 10 is a circular groove 31 (Fig. 2) in which an O-ring 33 is positioned for forming a gas-tight seal with the face of the cathode. Near the central portion of the isolator is a raised circular ridge 35 which fits into a circular groove 37 (Figs. 3 and 4) for aligning aperture 11 of isolator 10 with an aperture 38 in a cathode 40. As shown in Fig. 3, cathode 40 is of a circular configuration approximately 108 mm (4-1/4 inches) in diameter. The cathode is approximately 12.7 mm (1/2 inch) thick. The cathode, as shown in Fig. 3, is suitable for use in the analysis of conductive samples where the sample would be placed over aperture 38 and held tightly in place by atmospheric pressure forcing the sample against the surface of the aperture in view of the reduced pressure inside the glow discharge apparatus. Bounding aperture 38 is a raised annular ring 41 (best shown in Fig. 4) which has a planar sample contacting surface. Groove 37 surrounds ring 41. An O-ring or other elastic seal 45 can be placed in groove 37. In view of the heat generated in the cathode during the glow discharge process, a cooling water inlet 53 (Fig. 3) is provided through which water can enter into an internal conduit 55 leading to a cooling chamber 57 (Fig. 4) which surrounds the area occupied by the sample during the glow discharge process. Cooling chamber 57 is sealed by a disc 58. Water can exit chamber 57 through an internal conduit 59 and an external nipple 61. A plurality of spaced apertures 63 are provided in the cathode for insulated bolts to pass for attachment of the cathode to the anode of the glow discharge apparatus. Threaded apertures 65 are provided for the attachment of non-conductive isolator 10. Isolator 10 has a pair of apertures 67 (Fig. 1) which are bounded by sloping surface portions 69. A flat-headed screw, having sloping shoulders, can be conveniently screwed into apertures 65 in cathode 40 to attach the isolator to the cathode with the screws flush against the surface of isolator 10. Raised ridge 35 (Fig. 7) on the other face of isolator 10 is designed to fit into and interlock with circular groove 37 in the face of a cathode. By so fitting the cathode and isolator together, apertures 11 and 38 are aligned. Groove 37 would normally hold O-ring 45 when cathode 40 was being used as a DC Grimm-type glow discharge lamp with a conductive sample pressed against ring 41. O-ring 45 would provide a resilient seal between the sample and the cathode. Referring now to Fig. 5, a prior art direct current (DC) glow discharge source will be described having an anode 75 with an aperture 77 therein. Aperture 77 is bounded by a threaded wall 79. In order to cool the anode during operation of the glow discharge source, cooling water can enter through a port 81 and exit through a port 83. The glow discharge source has a cathode 85 which has an aperture 87 therein. An insulating member 91, having a tubular projecting portion 93, covers most of the surface 95 which bounds aperture 87 in cathode 85. A small annular portion of the cathode surface 97 is exposed adjacent to conductive sample 99. An anode insert member 101 has a threaded outer surface 103 which enables the insert member to be threaded into anode 75. Anode insert 101 has a projecting tubular portion 104 which passes through aperture 87 in the centre of the cathode and insulating member 91. Anode insert member 101 has an annular, flat surfaced, projecting ring portion 105 which is positioned close to conductive sample 99. The outer face 107 of cathode 85 has a circumferential groove 109 bounding the aperture in the cathode. An O-ring 111 or other elastic seal member can be placed in groove 109 to provide a gas-tight seal between sample 99 and the glow discharge source. In operating the DC glow discharge source of Fig. 5, a DC potential is applied between anode 75 and cathode 85. Insulating member 91 shields a major portion of face 95 of aperture 87 in cathode 85. A glow discharge can form between the anode insert 101 and exposed portion of sample 99. The glow discharge will generate ions of inert gas which will bombard the face of sample 99 causing a glow to form as electrons in excited atoms of the sample drop back to a lower energy state causing a glow to be emitted characteristic of the composition of the elements in sample 99. Sample 99 is in contact with cathode 85 and, as previously mentioned, is conductive and, in effect, forms a portion of the cathode of the glow discharge device. In order to analyze a non-conductive sample by glow discharge techniques, the preferred method is to use RF current to generate the glow discharge. Referring to Fig. 6, an RF glow discharge apparatus is shown which uses several of the components from the DC glow discharge device shown in Fig. 5 and, therefore, identical reference numerals will be used for the common elements. In general there is an anode 75 and a cathode 85 which are separated by an insulating member 91 which has a tubular portion 93 substantially covering the wall bounding aperture 87 in cathode 85. An insulating isolator 10 is attached to surface 107 of cathode 85 by suitable threaded fasteners (not shown). Isolator 10, as described above, is made from a suitable insulating material, such as a ceramic material, to provide electrical insulation and withstand the heat generated during the glow discharge process. On the side of isolator 10, facing surface 107 of cathode 85, is a circumferential projecting portion 35 which locks into groove 109 which formerly contained O-ring 111 (Fig. 5). By locking the isolator into this same groove, aperture 11 in the insert member is precisely aligned with the apertures through anode 75 and cathode 85. A circumferential groove 31 is provided on the face of isolator 10 for receiving an O-ring 33 to provide a gas-tight seal between the isolator and the interior of the glow discharge apparatus. On the opposite surface of isolator 10 is, preferably, a planar metallic ring 19 having a raised annular edge 21 for providing contact with the surface 135 of sample 137. Aperture 11 in isolator 10 is bounded by a non-conductive face 139 which terminates in a circumferential projecting edge 13. Edge 13 defines a groove 15 in which an O-ring 17 can be positioned to provide a gas-tight seal between surface 135 of sample 137 and the interior of the glow discharge apparatus. Annular metal contact surface 22 is approximately 0.125 mm (0.005 inch) higher than non-conductive edge 13. The raised metal ring provides a uniform contact with the face of the sample to be subjected to the glow discharge analysis. As previously mentioned in regard to Fig. 1, the surface of the isolator has a recessed channel in which an electrical conductor 27 can be inserted and attached to the edge of the metal contact member. Anode 75 has a threaded insert 153 which has an elongated central tubular portion which extends through aperture 87 in cathode 85 and substantially through aperture 11 in isolator 10 to position the end of the anode insert close to sample 137. A conductive annular collar 157 surrounds the end of the tubular portion of anode insert 153. The gap from anode insert collar 157 to face 135 of the cathodically connected sample should be approximately 0.12 mm. The concentric gap about collar 157 and insulating wall 139 should be the same distance. The concentric gap should be carefully machined. The end of the anode insert is made slightly larger and is then brought back to size by a facing tool working off the circumferential face in the insert member for positioning. By maintaining this close spacing the probability of plasma forming in the gap goes to zero. This area is also frequently referred to as a "dark space." It is important to note that isolator edge 13 substantially shields electrical contact member 19 from active ions during the glow discharge process. Raised ridge 13 effectively prevents the metal components of contact 19 from appearing in the glow discharge spectra of the sample being analyzed. In the operation of the device, and referring to Fig. 7, anode 75, cathode 85, isolator 10 and anode insert 153 are assembled together as shown in Fig. 6. Non-conductive sample 137, after having been given suitable surface polishing using several sizes of grit, is pressed against O-ring 17 and then the interior of the glow discharge apparatus is evacuated. The atmospheric pressure pushing against sample 137 will hold it in place against the electrical contact surface 22. The anode and cathode are at zero DC potential. A source of RF energy of approximately 100 watts at 13.56 MHz can be applied to conductor 27, electrical contact 19 and the face 135 of sample 137. The glow discharge will then form between the anode insert and the sample. During the glow discharge process, material is sputtered from the face of the sample. In order to remove this material a vacuum pump 157 (Fig. 7) is connected through a valve 159 to a conduit 161 which leads to the interior 163 of the glow discharge cell and the sample face. By pumping out approximately 50 ml per minute of Argon gas, the sputtered material will be drawn away from the sample face enabling a fresh sample surface to be continually presented for analysis. As mentioned above, the glow discharge is generated in area 155 of the anode insert tube. The analysis is carried out with a suitable spectrometer 170 which is focused, as shown by arrow 171, to look down through the bore of the aperture in the apparatus directly at the glow discharge region. It could be said that the spectrometer is bore sighted onto the glow discharge with the anode insert tube providing the bore. Referring now to Figs. 8-11, an alternative embodiment of interface assembly 209 is shown which can be attached to the cathode plate in a Grimm-type glow discharge lamp. Interface assembly 209 includes an isolator 210 made of a suitable non-conductive material, such as MACOR, a ceramic, and is approximately 44.5 mm (1.75 inches) in diameter and 6.35 mm (1/4 of an inch) thick at the widest central portion. The isolator has a central aperture 211 which is bounded by a raised edge 213. A groove 215 is provided adjacent edge 213 into which a circular, electrical contact member 219 is loosely positioned. Contact 219 has an external diameter approximately .25 mm (.01 inch) smaller than the outer diameter of groove 215 to allow for expansion of the contact during use. The dimensions and shape of contact 219 may be changed to vary the air gap in groove 215 between contact 219 and isolator 210 to vary the capacitive coupling between contact 219 and cathode 240. Contact 219 is manufactured from any suitable material such as copper. Contact 219 surrounds the edge 213 of isolator 210 and projects outwardly therefrom. Contact 219 is also received into the insulating material forming the isolator. Electrical contact 219 has a raised annular ring portion 221 which forms one of four sides for an elastic seal such as an O-ring 217. The top face 222 of the contact 219 projects outwardly approximately 1.65 mm (0.065 inch) beyond the top face of edge 213 to provide a uniform electrical contact with the face of a sample 216 (Fig. 13). While a circular electrical contact member is preferred, the contact can be of other configurations such as a rectangular contact pad having an aperture therein or multiple contact pads spaced about the aperture in the isolator. The important criterion is that the contact provides a substantially uniform contact concentric to the glow discharge region. Contact 219 is held in isolator 210 by a retainer 218 (Fig. 8). Retainer 218 is preferably secured to isolator 210 using nylon fasteners 272 such as nylon threaded fasteners. The retainer may be constructed of any suitable non-conductive material such as Teflon (Registered Trade Mark) or nylon. A recessed channel 223 is provided in the surface of the isolator 10 for an electrical contact (not shown) to be inserted such that it makes contact with conductive ring 219. A copper wire 227 can be inserted into channel 223 with the end of the wire either silver soldered to contact 219, or connected by a suitable threaded member, such as a screw (not shown). The opposite face of isolator 210 is substantially planar to contact an O-ring 245 (Fig. 13) to form a gas-tight seal with the face of the cathode 240. As shown in Fig. 12, cathode 240 is of a circular configuration approximately 110 mm (4-1/3 inches) in diameter. The cathode is approximately 12.7 mm (1/2 inch) thick. The cathode, as shown in Fig. 12, is suitable for use in the analysis of non-conductive samples and mates with a conventional anode 255. A central aperture 238 is provided in the cathode. A raised annular ring 241 has a planar contacting surface 242 for contacting isolator 210. Groove 237 circumscribes the interior of ring 241. An O-ring or other elastic seal 245 is placed in groove 237 to seal the joinder of cathode 240 and isolator 210. The opposite face of cathode 240 includes a circumferential groove 256 for receipt of a resilient gas sealing member, such as O-ring 258. The glow discharge lamp 250 for use with assembly 209 includes an electrically conductive spacer disc 244 positioned therebetween. The electrically conductive disc can be manufactured from any suitable material, and may be manufactured from the same material as anode 255 or cathode 240. Even though the cathode 240 and anode 255 are electrically connected by member 242, and are preferably connected to zero volts, the "anode" and "cathode" labels are retained in the description of this embodiment since these discs are physically positioned in the location of the anode and cathode of traditional Grimm-type lamps. Disc 244 includes a round, central opening which receives a precision spacer 246. The precision spacer is a circular member manufactured of a suitable material, and may for example be manufactured of a MACOR ceramic. The spacer includes a central aperture 248 for alignment with openings 238. In view of the heat generated during the glow discharge process, a cooling water inlet 253 (Fig. 12) is provided in anode 255 through which water can enter into an internal conduit (not shown) leading to a cooling chamber 257 (Fig. 13). Water can exit this area through an internal conduit (not shown) and an external nipple 261. A plurality of spaced apertures 263 are provided in the inner electrode for insulated bolts to pass for attachment of the cathode 240 to the anode 255 of the glow discharge lamp. Threaded apertures 265 (Fig. 12) are provided for the attachment of non-conductive isolator 210. Isolator 210 has a pair of apertures 267 (Fig. 10) which are bounded by sloping surface portions 269. A flat-headed screw (not shown), having sloping shoulders, can be conveniently screwed into apertures 265 in inner electrode 240 to attach the isolator to the cathode with the screws flush against the surface of isolator 210. Cathode 240 includes threaded apertures 268 for attachment of retainer 218. Isolator 210 also includes apertures 270 (Fig. 10) for passage of a threaded fastener 272 (Fig. 8) securing retainer 218 on the assembly. As mentioned above, in order to analyze a non-conductive sample by glow discharge techniques, the preferred method is to use RF current to generate the glow discharge. The insulating isolator 210 is attached to surface 242 of cathode 240 by suitable threaded fasteners (not shown). Isolator 210 is made from a suitable insulating material, such as a ceramic material, to provide electrical insulation and withstand the heat generated during the glow discharge process. The surface of isolator 210, facing surface 242 of cathode 240, is flat and abuts O-ring 245 to form a seal. The isolator 210 is positioned by the threaded fasteners (not shown) inserted into apertures 267 and threaded apertures 265 (Fig. 12). With reference now to Fig. 13, anode 255 has a threaded insert 283 which has an elongated central tubular portion 285 which extends through aperture 248 in cathode 240 and substantially through aperture 211 in isolator 210 to position the end of the anode insert close to sample 216. A cylindrical insulator tube 290, made of a suitable non-conductive material, such a ceramic, is positioned between the anode insert and members 240 and 210 and a portion of member 246. The cylinder provides an insulating wall. The gap from anode insert tip 292 to face 294 of the connected sample 216 should be approximately 0.12 mm. By maintaining this close spacing the probability of plasma forming in the gap goes to zero. This area is also frequently referred to as a "dark space." It is important to note that the edge of the isolator 210 and the tube 290 substantially shield electrical contact member 219 from active ions during the glow discharge process. They effectively prevent the metal components of contact 219 from appearing in the glow discharge spectra of the sample being analyzed. In the operation of the device, and referring to Fig. 13, anode 255, cathode 240, isolator 210 and anode insert 283 are assembled together as shown in Fig. 13. Non-conductive sample 216 is pressed against O-ring 217 and then the interior of the glow discharge apparatus is evacuated. The atmospheric pressure pushing against sample 216 will hold it in place against the electrical contact surface 222 on interface assembly 209. The anode and cathode are at zero DC potential. A source of RF energy of up to approximately 100 watts at 13.56 MHz can be applied to conductor 227, electrical contact 219 and the inner surface of sample 216. The glow discharge will then form between the anode and the sample in region 295 within the anode insert. During the glow discharge process, material is sputtered from the face of the sample. In order to remove this material a vacuum pump such as that shown in Fig. 7 may be utilized. It can clearly be seen that by use of the apparatus described the sample face being analyzed is the same face in contact with the source of RF energy. By viewing the same face, thickness effects of the sample, which previously complicated the process and tended to lead to inaccurate data, are eliminated. The analytical instrument looks directly at the glow discharge caused by active atoms from the face of the sample being analyzed. Although the invention has been described with respect to specific preferred embodiments thereof, many variations and modifications will become apparent to those skilled in the art.

Claims: An apparatus for use in the analysis of non-conductive samples by glow discharge technique comprising: a chamber for containing an ionizable gas; an anode (75) disposed in said chamber, said anode having an aperture (77) therein: a cathode (85) spaced from said anode, said cathode having an aperture (87) therein in alignment with said aperture (77) in said anode and forming with said aperture in said anode an axial passage (11); a non-conductive isolator (10,210) abutting said cathode for holding a sample (137) to be analyzed in line with said axial passage through said anode and cathode; an electrical contact member (19,219) on said isolator for contacting a side of a sample facing said axial passage; and a source of radio frequency energy for applying radio frequency energy to said electrical contact and to a sample in contact therewith for producing ions of an ionizable gas which will tend to move through said axial passage to contact the surface of a sample in contact with said electrical contact member. Apparatus according to claim 1 wherein said electrical contact member (19) is substantially shielded by said isolator (10) from gas ions in said axial passage. Apparatus according to claim 1 or claim 2 wherein said electrical contact member (19) comprises an electrically conductive planar ring disposed on a surface of said non-conductive isolator, the ring preferably having a raised annular sample contacting portion (21) bounding an aperture in said ring. Apparatus according to claim 3 wherein said non-conductive isolator has an aperture for alignment with said axial passage in said anode and cathode and an annular groove (15) spaced from and surrounding said aperture for receiving and supporting an elastic sealing member (17) for forming a gas-tight seal with a surface of said cathode. Apparatus according to claim 4 wherein said cathode has a first surface spaced from said anode and a second surface (107) on the opposite side of said cathode with an annular groove (109) therein surrounding said aperture in said cathode and said isolator has a raised circular portion (35) for interlocking with said annular groove in said second cathode surface for aligning said aperture in said isolator with said aperture in said cathode. Apparatus according to claim 4 or claim 5 wherein said isolator has a raised annular edge (13) bounding said aperture in said isolator which edge is spaced from and surrounding said annular raised portion of said conductive ring (19) for forming a groove for receiving and supporting an elastic sealing member (17) used to form a gas-tight seal with a surface of a sample. Apparatus as claimed in any preceding claim, in which: the cathode (85) has a circular groove (109) in the face (107) of said cathode member remote from said anode member (75), said groove surrounding the aperture (87) in said cathode member; an insulating member (91) is disposed between and electrically separates said anode member and said cathode member, said insulating member having a projecting tubular portion (93) defining an aperture in said insulating member, said projecting tubular portion extending through said aperture in said cathode member covering all but an annular portion of the wall defining said aperture in said cathode member; the electrical contact member (19) is on the surface of the isolator remote from said cathode member; and a tubular anode insert member (153) is added for attachment to said anode member and extending through said aperture in said insulating member and through said aperture in said isolator to a location closely adjacent to the exposed surface of said isolator. Apparatus according to claim 7 wherein said aperture in said anode member (75) is threaded and said anode insert member (153) is threaded for threaded attachment to said anode member. A method for the analysis of a non-conductive sample (137) by a radio frequency current induced glow discharge comprising: providing a glow discharge apparatus having an anode (75) and a cathode (85) with aligned apertures therein; attaching a non-conductive isolator (10,210) with an aperture therein to said cathode with the aperture in said isolator aligned with the apertures through said anode and cathode; providing an electrical contact member (19,219) on the side of said isolator remote from said cathode; affixing the surface of a sample (137) to be analyzed to said electrical contact member and so as to close said aperture in said isolator with the same surface of said sample; generating a glow discharge adjacent to the surface of said sample in electrical contact with said contact member; and analysing said glow discharge by viewing through said aperture in said anode and cathode. A method according to claim 9 which includes applying a vacuum source (158) to the vicinity of said glow discharge to remove sputtered sample material to provide a clean sample surface for analysis.

Inventors: CASPER TED JOSEPH, MITCHELL JOEL CARLTON, CASPER, TED JOSEPH, MITCHELL, JOEL CARLTON
Applicants: LECO CORP, LECO CORPORATION
Application year: 1994
Main classification: G01N21
IPC codes: G01N21, G01N27, H01J49